1. Field of the Invention
The present invention relates to signal control apparatus. The invention is applicable for example to a loudness control circuit in an audio equipment such as a radio, player and deck provided with an electronic volume controller. The loudness control circuit embodying the present invention can automatically adjust the loudness of such audio equipment.
2. Description of the Prior Art
In the field of such as audio equipment and electric signal receivers, a signal control apparatus is known which can selectively choose the characteristic of a signal to be outputted.
For instance, in audio equipment such as radio receivers and players, volume control has been effected heretofore by means of a mechanical method of turning a volume controller. Recently, however, with the advent of various electronic devices, volume control is often effected by means of an electronic volume controller using for example a voltage controlled type variable gain circuit. Use of such an electronic volume controller enables the variation of sound intensity by manipulation of a key without necessitating the provision of conventional mechanical movable parts, thereby ensuring a high reliability and an improvement on operation performance.
Loudness control is commonly used, when the sound intensity is set at low levels in the middle of the night for example, to compensate for the reduced response of the ear to low and high frequency components.
As the volume controller in a conventional mechanical volume control circuit is turned to adjust the sound intensity, loudness control is effected whose characteristic automatically and proportionally varies with a change in sound intensity in accordance with an output from the loudness control tap of the volume controller.
However, a change in resistance of the mechanical volume controller at various setting positions affects the loudness characteristic. It is therefore difficult to obtain an ideal loudness characteristic suitable for auditory sensation.
In contrast with the mechanical volume controller, loudness control using an electronic volume controller has been used heretofore in the following way. As shown in FIG. 1, an audio signal is amplified by both flat circuit 10 and loudness circuit 12 so that two independent audio signals are produced, one having a flat frequency characteristic and the other having a loudness frequency characteristic. The output signals from the flat circuit 10 and loudness circuit 12 are coupled to a switching circuit 14 where the signals are selected in response to a switching signal S. That is, when the sound intensity is high, the output signal from the flat circuit 10 is outputted from the switching circuit 14, while at a low sound intensity, the signal from the loudness circuit 12 is outputted from the switching circuit 14.
Such loudness control has been found to be unsatisfactory in that it is cumbersome to manipulate not only the volume controller but also an ON/OFF switch for the loudness control, and furthermore in that unnatural sound with its low or high frequencies excessively emphasized may sometimes be produced at a certain sound intensity since the loudness characteristic does not vary in proportion to a change in sound intensity.
As described above, the conventional signal control apparatus selectively changing the characteristics of signals to be outputted has a problem associated with an continuous characteristic change.